Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image for forming an electrophotographic image, and a process for producing the same.
Description of Related Art
Recently, in the field of electrophotographic image forming apparatus, toner for developing an electrostatic latent image (hereinafter also referred simply as “toner”) suitable for electrophotographic image formation has been developed in response to the demand of the market. For example, as toners for providing a high-quality image, such toners have been required that have a sharp particle size distribution, i.e., toners that have uniform toner particle diameters. In such toners, each individual toner particle exhibits uniform development behavior and thereby the reproducibility of fine dots remarkably improves. However, it is not easy to make the toner particle diameter distribution sharper with conventional toner production methods using the pulverization method. To address this problem, the emulsion aggregation method has been used as a method capable of controlling toner particles to have any desired shape and particle size distribution. The emulsion aggregation method is a method of obtaining toner particles wherein resin microparticles and colorant microparticles, together with release agent microparticles as necessary, are aggregated by addition of an aggregation agent or by pH control while mixing and stirring, and the aggregated microparticles are further fused together under heating.
Further, the development of low-temperature fixable toners, which can be fixed with lesser energy, has been in progress from the viewpoint of energy-saving. In order to lower the fixing temperature of toner, it is necessary to lower the melt temperature or melt viscosity of the binder resin. However, when the glass transition point or molecular weight of the binder resin is lowered for the purpose of lowering the melt temperature or melt viscosity of the binder resin, new problems arise such as low toner high-temperature storability and/or low separability of sheet from the fixing member upon fixation (hereinafter simply referred to as “separability”).
In order to achieve both low-temperature fixability and high-temperature storability, it has been common practice to allow toner particles to have a core-shell structure (see, e.g., Japanese Patent Application Laid-Open No. 2012-189940). That is, the formation of a shell layer composed of resin that exhibits excellent high-temperature storability and high softening point on the surface of core particles composed of binder resin that exerts excellent effects in low-temperature fixability can provide both low-temperature fixability and high-temperature storability. In particular, such a core-shell structure can be easily formed when the emulsion aggregation method is used.
As such a toner having toner particles with a core-shell structure, toners have been developed that include a polyester resin as the resin constituting the shell layer. Since the polyester resin can be advantageously easily designed to have a lower softening point while maintaining a higher glass transition point as compared to styrene acrylic resins, it is possible to obtain a toner excellent both in low-temperature fixability and high-temperature storability by using a polyester resin as the resin for the shell layer.
However, when a styrene acrylic resin is used as the resin for the core particle and a polyester resin is used as the resin for the shell layer, high-temperature storability is insufficient since lack of affinity between the styrene acrylic resin and the polyester resin makes it difficult to form a thin and uniform shell layer. In addition, difficulty in controlling the shape of toner particles due to the unlikeliness of fusion between the core particle and shell layer makes it difficult to form a shell layer with uniform surface and thus to form toner particles having a dense and smooth surface, resulting in the shell layer to come off due to its inferior fracture resistance by the stirring of the toner in a developing device during continuous printing. As a consequence, the electric charge amount greatly varies and hence unwanted noise occurs in the resultant image resulting in low image quality.
In order to solve these problems, for example, Japanese Patent Application Laid-Open No. 2013-109246 discloses a toner having a core-shell structure that includes an acrylic-modified polyester resin as the resin for the shell layer. The use of acrylic modified polyester resin as the resin for the shell layer improves its affinity for the styrene acrylic resin constituting the core particle, and thus it becomes possible to form a shell layer having a surface with a certain degree of uniformity.
However, since the release agent (wax) has high hydrophobicity, it is likely that migration of the release agent is blocked by the shell layer during heat fixing and therefore the release agent fails to exude as far as to the surface of the fixed image and as a result sufficient separability is not achieved.